It is well known that faults in optical fibres can be located by an optical time domain reflectometer (OTDR). An OTDR typically launches a pulse of light into an optical fibre, and backscattered light is monitored for abrupt changes indicative of a loss or fault. The distance of the loss or fault from the launch end of the fibre can be determined from the time interval between the launch and the return of the backscattered peak. Once a period of time sufficient to receive all detectable backscattered light has passed, a further pulse may be launched into the fibre. The pulse width may be varied for different dynamic range or resolution requirements. Thus, for a given amplitude, an increase in the pulse width enables a greater length of fibre to be monitored, that is to say it increases the dynamic range of the OTDR. The dynamic range of an OTDR is the loss after which an event, backscatter or reflection can still be detected.
The OTDR is an extremely useful form of optical test equipment since, from connection to a single end of an optical fibre network, the location of losses and reflections can be determined, and their amplitude measured, to a high degree of accuracy. For loss measurements, both point-losses and end-to-end fibre or network losses can be measured. In duplex networks, the amplitude of any reflections is important, since these may cause crosstalk. In some known schemes it is possible using wavelength division multiplexing (WDM) techniques, to take these measurements at a particular wavelength i.e. 1650 nm whilst the network is carrying data at another wavelength i.e. 1550 nm.
As of late, there has been a demand for an OTDR system that conveniently and inexpensively allows for the reliability testing of individual optical fibres within optical networks of optical fibers. Methods used heretofore in conventional OTDR are not suitable, since the Rayleigh back-scattered light from different branches cannot be differentiated at the OTDR. Notwithstanding, recently, several methods based on multi-wavelength OTDR have been proposed in:
a paper by I. Sankawa, "Fault Location Technique for In-Service Branched Optical Fiber Networks", published in IEEE Photon. Technol. Lett., vol. 2, no 10, pp 766-768, 1990; PA0 a paper by Y. Koyamada, et al, entitled "Recent Progress in OTDR Technologies for Maintaining Optical fiber Networks", IOOC'95, Paper FA1-4, Hong Kong, 1995. PA0 a paper by K. Tanaka, et al, "Measuring the Individual Attention Distribution of Passive Branched Optical Networks", IEEE Photon. Technol. Lett., vol. 8, no. 7, pp. 915-917, 1966; and, PA0 a paper by M. Shigehara, et al., "Optical Fiber Identification System Using Fiber Bragg Gratings", Technical Digest, OFC'96 Paper WK13, San Jose, 1966; all of which are incorporated herein by reference.
However, these proposed OTDR systems require a wavelength-tunable pulsed light source for the test signals and thus impose high cost and complexity. For the schemes proposed by Sankawa, and Koyamada et al., strong monitoring signals from the wavelength-tunable light sources may deplete the gain of the in-line optical amplifiers and result in system penalty; whereas this invention, in contrast provides an in-service fault detection system using optical amplifiers on passive branched optical networks (PBONs) and on a concatentated link without requiring an extra light source.
Two known systems that address OTDR in optical networks are disclosed in U.S. Pat. Nos. 5,491,573 and 5,491,574 in the name of Shipley. Another United States patent in the same field is U.S. Pat. No. 5,329,392 in the name of Cohen. In view of the limitations of these aforementioned inventions, it is an object of one aspect of this invention to provide a novel passive surveillance scheme for fault identification in optically-amplified PBON using fiber Bragg gratings (FBG) without using any extra light source for the test signals. The invention further obviates the use of a tunable OTDR pulsed light source.
It is a further object of the invention to provide an inexpensive device for testing an optical network of waveguides in the presence of traffic.
More generally, it is an object of this invention to provide a passive surveillance system that is compatible with the use of optical amplifiers and which utilizes otherwise unused spectrum provided by a rare earth doped optical amplifier.